In closed mixers, especially of the type having meshing rotors, the mixing and `cutting` action takes place mainly, or almost entirely, between the opposing surfaces of the two rotors.
In order to ensure good mixing, the distance or `gap` between the surfaces of the rotors across which the working process is carried out is highly important in these mixers and, therefore, there is a considerable advantage in being able to choose this distance depending upon the type of manufacturing process to be carried out.
As time goes by, wear and tear of the working surfaces of the rotors also takes place, resulting in ever increasing play between the rotors to the extent that the working features of the machine are modified, the mixing becoming progressively worse until it is necessary to replace the rotors or to overhaul them, refacing the surfaces by means of welding, so as to obtain the original geometrical form.
These actions are complicated and costly and the machine must be kept inoperative for a long time in order to allow the rotors to be dismounted, to be replaced or to be overhauled.
It is therefore important to provide a mixer, especially of the type having meshing rotors, in which it is possible both to vary the gap between the rotors to suit the various conditions of use and to allow for the wear and tear of the rotors to be compensated, thereby allowing these rotors to be used for a long time before overhauling and rebuilding the worn surfaces, thus ensuring the best working condition over long periods.
These results are achieved by the present invention, which provides a closed mixer having rotors whose axial separation is adjustable, comprising an external casing housing a mixing chamber containing two rotors with projecting lobes operating at a limited distance from one another, and caused to rotate by a motor unit via transmission means, wherein the rotors are equipped with end shafts, having a relatively small diameter, rotatably supported within opposite heads of the external casing by means of supports housed within elements which can move and be locked in a number of positions. The elements are moved to vary the distance between the axes of the rotors. Holding means are provided between the rotors and the heads of the chambers across which they extend in order to ensure that the axial separation of the rotors is maintained in the various positions.
In a particular embodiment, the rotors have meshing lobes, whose complementary profiles define throughout their rotation one or more regions at short distance between the mutually juxtaposed working surfaces.
According to one embodiment of the invention, the moving elements housing the supports for the rotors comprise, for each head, two rings rotatably carried within the respective head of the external casing of the mixer, and housing the supports for the rotors in an eccentric manner. Controls are provided for causing rotation of the rings, thus varying the position of the axes of the rotation of the rotors. Means are also provided for locking the rings in the wanted positions.
The means for rotating the rings may suitably consist of a worm interposed between the rings and meshing with worm wheel teeth extending around the perimeter of the rings themselves. The worm is rotated by a suitable control via a speed reducing unit.
The locking means may comprise indentations, cut-outs, holes or the like provided on the surfaces of each eccentric ring, in one or more of which there is inserted either a peg or a locking key secured to the external casing of the mixer, or locking screws adjustable in different positions, or the like. The holding means provided between the rotors and the heads of the chamber may comprise front sealing rings acting against flat surfaces which join the ends of the rotors to their respective support shafts, the sealing rings, which are carried by the head walls of the mixing chambers, having a larger internal diameter than the diameter of the support shafts, so that the support shafts are allowed lateral displacement, as required, internally. Alternatively, the holding means provided between the rotors and the heads of the chamber may comprise radial sealing rings acting on the cylindrical surfaces of the support shafts for the rotors carried by extensions of the eccentric rings and rotating together with them within the head walls of the mixing chambers.
In a further embodiment of the present invention, the control means for the rotation of the eccentric rings comprises a shaft tangential to the circumference of both rings carrying two worm screw parts, each formed as a screw thread directed in an opposite direction from the other end engaging with corresponding teeth extending around the perimeter of the respective eccentric ring, and caused to rotate by means of a control via a motor unit.
According to yet a further embodiment, the movable elements may comprise slides within which the support shafts for the rotors are housed, it being possible to alter the position of the slides in relation to one another by means of controllable actuating means.
The movable elements may also consist of polygonal bushes within which there are carried, in an eccentric manner, the support shafts for the rotors, the bushes being inserted within corresponding polygonal seats in the heads of the external casing of the mixer, wherein variation of the distance between the axes of the rotors is achieved by extracting the bushes from their seats and subsequently inserting them in a relatively rotated position.